R2(config-router)#aggregate-address 11.11.11.0 255.255.255.0 ?
advertise-map Set condition to advertise attribute
as-set Generate AS set path information
attribute-map Set attributes of aggregate
nlri Nlri aggregate applies to
route-map Set parameters of aggregate
summary-only Filter more specific routes from updates
suppress-map Conditionally filter more specific routes from updates
<cr>
R3#show ip bgp
BGP table version is 4, local router ID is 2.2.2.3
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*> 11.11.11.0/25 2.2.2.2 0 2 1 i
*> 11.11.11.0/24 2.2.2.2 0 0 2 i
*> 11.11.11.128/25 2.2.2.2 0 2 1 i
R3#show ip bgp 11.11.11.0 255.255.255.0
BGP routing table entry for 11.11.11.0/24, version 3
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Flag: 0x820
Not advertised to any peer
2, (aggregated by 2 2.2.2.2)
2.2.2.2 from 2.2.2.2 (2.2.2.2)
Origin IGP, metric 0, localpref 100, valid, external, atomic-aggregate, best
Notice that the AS_PATH for the aggregate route shows just the ASN 2 even though the 11.11.11.0 network is actually originated from AS1 (R1). The more specific prefixes actually retains the AS_PATH {2 1}. The "as-set" option enables the routes to retain the AS_PATH information along with other attributes (no-export, no-advertise communities and etc) and it could be used to prevent loops.After I apply the as-set option notice the AS_PATH on R3 for 11.11.11.0/24 is now {2 1}.
R2#conf t R2(config)#router bgp 2 R2(config-router)#aggregate-address 11.11.11.0 255.255.255.0 summary-only
as-set
R3#show ip bgp
BGP table version is 21, local router ID is 2.2.2.3 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S Stale Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path *> 11.11.11.0/25 2.2.2.2 0 2 1 i *> 11.11.11.0/24 2.2.2.2 0 0
2 1
i *> 11.11.11.128/25 2.2.2.2 0 2 1 i
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